1. Field of the Invention (Technical Field)
The present invention relates to the field of photovoltaic modules, in particular to photovoltaic modules amenable to automated assembly for utilization in a fixed or sunlight-tracking array.
2. Background Art
Photovoltaic (PV) modules, also referred to as PV “panels” or “collectors” are used to collect sunlight for conversion into useful forms of energy, particularly for the generation of electricity for use in industrial applications and homes. The typical PV module has remained relatively unchanged from its original configuration of approximately thirty years ago. FIGS. 7a and 7b are representative of a prior art PV module, where FIG. 7a shows a perspective view of the prior art PV module and FIG. 7b shows an exploded view of the module of FIG. 7a. Referring to FIG. 7b, the prior art PV module is comprised of an array of sliced crystalline or semicrystalline silicon solar cells 300. Each cell 300 converts photons that strike the cell into a flow of electrons. The electrons flow across the cell on a grid pattern (not shown) as electrical current, the current being channeled to a pair of redundant parallel bus bars 302, 302′ also referred to as “interconnects”, on each cell. The electrical current then flows through the bus bars 302, 302′ from one solar cell to the next. Each cell 300 is thus oppositely polarized at opposing ends creating a relatively small voltage across the cell, typically approximately one-half of a volt. Because of the small voltage across each individual solar cell. PV modules comprise a plurality of solar cells connected in series to attain useful voltages such as 120 volts or 240 volts. Electrical current flowing through each solar cell is approximately five to six amperes.
The solar cells of a PV module are electrically connected end-to-end in series in a “string” so that the voltage from each cell is added to the voltage of the adjacent cell. These strings, or rows, of cells are then placed adjacent to one another, each row of cells being electrically connected in series with the adjacent row by a bus bar as indicated at 304. The resulting array is therefore one continuous series of solar cells arranged in a serpentine manner as shown in FIG. 8. Sometimes, the PV strings are connected in a series/parallel combination.
This array of solar cells is encapsulated within a transparent polymer 306, 306′ to seal the solar cells from the elements. The encapsulant is in turn sandwiched between tempered glass 308 on the front surface where photons strike and a polymer sheet 310 on the back surface. This panel 310 provides a moisture barrier. An aluminum frame 312 protects the perimeter of the tempered glass 308 on the front surface of the PV module and also provides a mounting frame for the module. A junction box 314 provides the electrical connection to the module.
Fabrication of the individual solar cells and the assembly, or “stringing”, of cells together in series is typically performed in an automated fashion. The remaining manufacturing tasks are less amenable to automation. In particular, the connection of one string of solar cells to the next in a serpentine fashion requires an assembly technician to manually connect the end of one string to another via a suitable conductor. In particular, there are three facets of the prior art PV module configuration that make automated assembly difficult: 1) Every other string must be turned around so that its electrical polarity forms a continuous serpentine series electrical circuit. Turning the strings requires a large amount of factory space and is fraught with opportunity to damage the circuit. 2) Placing a turned string next to the adjacent string requires precision and the ability to ensure that it has been placed correctly. If the strings are not spaced apart adequately, solar cells in adjacent strings can be damaged; however, if the strings are not spaced close enough together, the array will not fit into the frame 312. 3) A terminal bar must be placed at the ends of adjacent strings and soldered into the overall circuit to make the series chain complete. To perform these tasks automatically requires a large capital expense in the form of robots or other programmable equipment capable of performing precise and complex maneuvers. The result has been that the expense of manufacturing PV modules renders large-scale production of PV modules cost-prohibitive.
Another limitation of prior art PV modules occurs when in use in the field. Most PV modules are fixed in position relative to the angle of incidence of the sun. When sunlight strikes the module at a shallow angle, the conversion of photons to a flow of electrons is less than optimal. PV modules convert photons to electrons most efficiently when sunlight strikes the PV module normal to the surface of the PV module. Prior art PV modules have attempted to address this issue by mounting the entire array of solar cells on a fulcrum and causing the array to pivot on the fulcrum in accordance with movement of the sun. One limitation of this configuration is the cumbersome nature of pivoting the entire PV array. Space limitations in a residential application may prohibit movement of the array. The torque and control means required to pivot the array especially under winder wind-loading can also make such an array cost-prohibitive. Another limitation of this configuration is shadowing. At the end of a day, certain obstructions, such as houses or trees, can produce a shadow on modules. Once any one solar cell entirely shadowed it ceases its electrical output, thereby opening the series circuit and inhibiting the entire series string from producing current.
Pivoting individual multi-string modules about their own axes is limited because adjacent modules shadow the cells along the extreme edges of the individual modules at the beginning and end of the day. Once one string is in shadow, the entire system ceases the flow of electrons because they are in series. Utilities have built multiple adjacent tracking systems wherein significant space between arrays is allotted to minimize the effect of shadowing. Such liberal use of space is generally not acceptable on residential and individual buildings.
The present invention for a combination PV, thermal solar panel and roofing material overcomes the limitations of the prior art and eliminates the manufacturing issues described above. The present invention for a PV module is assembled by an automated process, resulting in reduced assembly cost as compared to prior art PV modules. The present invention can be readily incorporated into a sunlight-tracking apparatus wherein each PV module is pivoted about a longitudinal axis in relation to the angle of incidence of sunlight striking the module. The present invention can also be used for the separate functions of collecting sunlight, collecting excess radiated heat from the sun and channeling it to an appropriate location, and providing a roofing material.